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Posts published in “Math”

花花酱 LeetCode 611. Valid Triangle Number

Problem:

Given an array consists of non-negative integers, your task is to count the number of triplets chosen from the array that can make triangles if we take them as side lengths of a triangle.

Example 1:

Input: [2,2,3,4]
Output: 3
Explanation:
Valid combinations are: 
2,3,4 (using the first 2)
2,3,4 (using the second 2)
2,2,3

Note:

  1. The length of the given array won’t exceed 1000.
  2. The integers in the given array are in the range of [0, 1000].

Idea:

Greedy

Time Complexity:

O(n^2)

Solution:

 

花花酱 LeetCode 204. Count Primes

Problem:

Count the number of prime numbers less than a non-negative number, n.

Idea:

Sieve of Eratosthenes

Time Complexity:

O(nloglogn)

Space Complexity:

O(n)

Solution:

C++

Java

Python3